1. Field of the Invention
Certain embodiments of the present invention relate to composite materials and, in particular, woven fiber composites reinforced through-the-thickness with carbon nanotubes.
2. Description of the Related Art
Composite materials have been developed to meet increasing demands for materials possessing a broad array of desirable properties. Composites are material systems which combine two or more distinct materials, each with its own distinctive, desirable properties, to create a new material with properties that may not be present, or to the same extent, in the components alone. Composite materials, broadly, possess at least two phases—a reinforcement and a matrix. The reinforcement is a material which is embedded within the matrix. In general, the reinforcing material and the matrix material comprise any combination of metals, ceramics, or polymers. The utility and versatility of composites has resulted in their use within a wide variety of applications, ranging from aircraft and marine structural components to sporting goods.
Significant research has been directed to composite materials which are reinforced in one- and two-dimensions (1-D, 2-D). 1-D and 2-D continuous-fiber reinforced composites (CFRCs) employ long fibers which substantially span the length and/or width of the composite material. In certain CFRCs, these fibers may take the form of unidirectional tapes or fiber cloths, where small diameter filaments are woven to form cloths having fibers which extend in predetermined orientations, such as 0°/90° and ±45°. This allows the composite to be constructed for a specific loading condition, placing the relatively strong fibers in a position where they carry the majority of the applied stress.
However, an inherent weakness of 1-D and 2-D CFRCs is their interlaminar and intralaminar properties. Interlaminar and intralaminar refers to processes which happen between or within planes of the fibers, which are generally stacked through the thickness of the composite. As 1-D and 2-D CFRCs lack reinforcement out of the plane of the fibers, they possess little resistance to out of plane deformation. As a result, these CFRCs possess low interlaminar fracture toughness, and interlaminar failure, such as delamination, may occur at relatively low levels of applied stress under various loading conditions.
To mitigate this weakness, 3-D composite architectures, with fibers running both in 2-D in-plane and orthogonal to the fiber plane, have been explored. However, attempts to develop these 3-D reinforced composites employing braided or through-the-thickness stitched fibers have met with mixed success. Investigations of composite laminates with 3-D braided reinforcements have found improvements in damage tolerance but also determined that the braided reinforcement and the non-normal orientation of the braided fiber with the 2-D in-plane fiber results in low in-plane strengths. The low in-plane strengths limit the applicability of the 3-D braided composites to specific applications and geometries. In the case of stitching, the out-of-plane reinforcing fibers can be orthogonal to the 2-D reinforcing fibers; however, the in-plane mechanical performance of the stitched composites depend critically on the stitch pattern. In practice, stitching has been found to shorten the tensile fatigue life of the composite and stitched laminates are reported to have tension and compression strengths of approximately 20-25 percent lower than the strengths of unstitched laminates.
To address these deficiencies, composite systems employing carbon nanotubes as a through-thickness reinforcement are now being developed. Carbon nanotubes (CNTs) are quasi-one dimensional, nearly single crystalline (axially), hollow, graphitic carbon structures. Their combination of high aspect ratio, small size, and excellent mechanical properties, coupled with low density, and high electrical conductivity make these materials good candidates as reinforcements in 3-D reinforced composites. Different researchers have reported significant improvements in the in-plane mechanical properties of carbon nanotubes (CNTs) reinforced nanocomposites compared to the unreinforced counterparts. However, lack of control of the orientation of the nanotubes and their dispersion is still a major challenge and indeed restricts their usage in structural applications
These deficiencies in the design of current 3-D reinforced composites illustrate the need for improved systems and methods for through-thickness reinforcement of 2-D continuous-fiber reinforced composites, and other improvements discussed below.